Electronic backplanes are utilized in circuit platforms for telephony, computer and other electronic systems to permit various circuit boards, cards or other circuit modules to be interconnected or to be connected to external circuitry. In the following, a circuit platform or platforms will be considered to be a backplane and associated card cage or other circuit module support elements. In some instances, the term may also encompass the circuit modules mounted in the support. A platform may accommodate a varying number of circuit modules depending on application.
Typically, such backplanes have one or more connectors for each circuit module, each connector having a plurality of terminals which on one side mate with matching terminals on the corresponding circuit board and which are hard-wired on the other side to terminals in the backplane or through other connectors to external circuitry. Multiple connectors may be provided for each circuit module. A runner in the platform facilitates insertion and removal of the circuit modules, the connector(s) of the circuit module mating with the backplane connector(s) when the circuit module is fully inserted.
Platforms of this type are useful for permitting circuit modules of a particular type to be easily removed and replaced for preventive maintenance, to replace a defective circuit module, or to replace an existing module with an updated or improved module. However, such backplane platforms offer little flexibility where it is desired or required to use circuit modules of different types, having circuitry interconnects which are sufficiently different so as to require different connectors and different wiring on the backplane. In particular, where the circuit modules are switching circuits which are part of a central office telephone switching system, the I/O connections for the circuits may use T1 cable with 56 connections per cable and two cables per module, each cable being formed of twisted pair 22 gauge wire, may use DS3 signal cables which involve six coaxial cables per module, may utilize fiber optic cables with, for example, six fiber optic cables per module, or may utilize some other type of connector and wiring. Further, it may be desired, to have mixed circuit modules on a single platform, with some modules being of one type and some of another type.
In any event, when changes are to be made, it is preferable that it be possible to effect such changes on line without causing an interruption in service for other modules on the same platform. However, it is very difficult to change the backplane wiring for a hard-wired backplane because of space limitations in both the front and the rear of the platform, and it is virtually impossible to change to a different type of I/O connector. Performing such backplane rewiring in the field without interrupting service on adjacent circuits is also very difficult, but is desirable since any contact with adjacent circuits could cause failures in such circuits and might also result in shock, burns or other injuries to a person doing such replacement.
Heretofore, the difficulty of running new cables of a different type into a backplane once the backplane has been installed has been dealt with in some instances by running cables of different types into the backplane, connecting the cables to be utilized, and tying off the remaining cables until such time as their use may be required. However, this procedure only partially simplifies the field replacement problem and results in a significantly more expensive and time-consuming initial installation, both in terms of labor and materials. Further, too many unterminated lines in a backplane may degrade signals and thus performance. There are also space limitations in a backplane which may make installation of extra cables difficult and complicate working on the backplane with the extra cables. Finally, since platforms may be used for many years, frequently in excess of ten years, desired cables, for example fiber optic cable, may not have existed at the time the backplane was installed.
A need, therefore, exists for a more flexible backplane system which permits connectors to be easily replaced in the field to accommodate different types of circuit modules, and which permits any rewiring to be done from the front of the platform where wires can be gotten at easily and without danger of damaging adjacent circuits or of injury to a person doing the installation. Such a technique should permit the path for any replaceable backplane elements or modules to be carefully controlled so as not to cause damage to adjacent circuit modules which are in use, while assuring that replacement components maneuver around structural portions of the platform and that replacement backplane modules are accurately positioned when fully installed.